1. Field Of The Invention
The present invention relates generally to fuel composition for a motor vehicle, and more particularly to a method and apparatus for determining the percent alcohol content of a fuel used in a motor vehicle capable of operating on more than one type of fuel.
2. Description Of The Related Art
Environmental and energy independence concerns have stimulated the development of alternative transportation fuels, such as alcohol-based fuels, for use in automobiles. Alcohol fuels include methanol and ethanol. A flexible fuel vehicle capable of operating on gasoline, or alcohol fuel, or any mixture of the two fuels, is one design that meets this demand. Modifications to the engine are necessary when operating on different fuels, because of the different characteristics of each fuel. For example, an engine operating on methanol or M85 (a blend of 85% methanol and 15% gasoline) requires approximately 1.8 times the amount of fuel relative to gasoline at stoichiometry due to a lower energy content of the methanol.
One means for identifying the ratio of gasoline to alcohol fuel present in the fuel tank is to use a fuel composition sensor. The fuel composition sensor may be located in or near the fuel tank, and its function is to determine the percentage of alcohol in a gasoline/alcohol fuel mixture.
For example, commonly assigned U.S. Pat. No. 5,119,671 hereby expressly incorporated by reference, discloses a fuel composition sensor that measures the percent alcohol content of the fuel and relays that information to an engine controller or the like so that dependant variables such as spark timing and air/fuel ratio can be adjusted accordingly.
Commonly assigned application, U.S. Ser. No. unknown, filed on the same day as the subject application and entitled "Method For Determining Fuel Composition", provides a method for determining the percent alcohol content of the fuel when the fuel composition sensor is not functioning properly, by utilizing the oxygen sensor feedback loop to identify changes in air/fuel ratio due to a changing fuel mixture.
In a typical automotive vehicle, a microprocessor, contained in an engine control unit (ECU), receives inputs from a number of sensors, processes the information from the sensors, and sends out instructions to selectively adjust various parameters. The process may be open loop or closed loop. The ECU stores the information it receives in two types of memory, erasable read only memory (ROM) and non-erasable random access memory (RAM).
The engine initially operates open loop until the engine attains a certain operating temperature, and then closed loop. A closed loop strategy can be described as an iterative process whereby the system output is also a factor of the input for next iteration. First, the parameters to be controlled are initialized to predetermined values and inputs are received based on current conditions; secondly, the information is processed; and finally, an output is produced. The output information then is available as an input parameter for the next iteration. This adjustment sequence is commonly called a feedback loop, whereby a system is able to learn about the environment it is operating within and make necessary adjustments.
An open loop strategy is similar to a closed loop system, except that the feedback loop is eliminated. In a vehicle it is generally used from the time the engine is cold-started until the engine has warmed up.
Similarly, adaptive memories are closed loop systems widely known in the automotive industry. Adaptive memories are initialized in RAM memory with a preset value regarding some parameter of the control system. These stored parameters are then continuously updated and adjusted as engine operating conditions change, and can be "looked up" and used by the engine controller when similar conditions are encountered.
Air/fuel ratio in internal combustion engine design is typically considered to be the ratio of mass flow rate of air to mass flow rate of fuel inducted by an internal combustion engine to achieve conversion of the fuel into completely oxidized products. The chemically correct ratio corresponding to complete oxidization of the products is called stoichiometric. If the air/fuel ratio is less than stoichiometric, an engine is said to be operating rich, i.e. too much fuel is being burned in proportion to the amount of air to achieve perfect combustion. Likewise, if the air/fuel ratio is greater than stoichiometric, an engine is said to be operating lean, i.e. too much air is being burned in proportion to the amount of fuel to achieve perfect combustion. Alcohol fuels have a higher or leaner air/fuel ratio than gasoline at stoichiometric, so that the engine must be compensated for in the rich direction a the percentage of alcohol in the fuel increases.
The oxygen sensor is located in the exhaust system and detects the amount of oxygen contained in the exhaust products leaving the engine through the exhaust system. The oxygen sensor then generates an input signal to the ECU based on the measured air/fuel ratio. The ECU processes the various input signals and its output signal varies the fuel injector's pulsewidth to adjust the fuel flow rate in an amount necessary to achieve the desired air/fuel ratio.
Under normal operating conditions, fuel vapors will collect inside the fuel tank. These vapors are temporarily stored inside vapor storage canisters containing activated charcoal, typically called purge canisters. Because of the limited storage capacity of the purge canister, it is periodically necessary to draw fresh air through the canister to remove the fuel vapor to the engine where it is burned. The purge solenoid utilizes input from the ECU to control the purge of vapors from the canister. An example is shown in U.S. Pat. No. 4,821,701, "Purge Corruption Detection", which is hereby expressly incorporated by reference.